In order to increase the productivity of oil and gas wells, or to bring back into production wells that have essentially gone dry, it is common practice to conduct a procedure known as fracturing the well. In such a procedure, fluids known as fracturing fluids, are injected into the well at very high hydrostatic pressures. The fluids are typically viscous gels, and act under pressure to open pores and cracks in the formation, thereby to increase the overall permeability of the well. Typically, the fluids also are used to transport proppants, such as sand or ground walnut shells into the cracks and pores, thereby to ensure that the pores and cracks formed during fracturing remain open under the lower pressures that will exist after fracturing, when the well is producing.
The fluid that has been used to fracture the formation is removed by the introduction of viscosity lowering agents into same, which permit the fluid to be more easily pumped from the formation. These agents are known as breakers because they tend to break down the fracturing gel. Breakers can act on a gel in a number of ways, such as by the random oxidation of polymers to shorten the chain length thereof. In the present invention, a breaker is utilized to adjust the pH of a gel, to break same by hydrolysis. The pH of the resulting formulation in accordance with the present invention can generally range from between about 1.0 to about 2.0. However, most preferably, the pH will range from 1.4 and below.
The fracturing fluid of the present invention is a phosphate alkyl ester gel. It is known that a gel can be produced by mixing trivalent cations, such as aluminum, with a phosphate alkyl ester. However, gels obtained with known phosphate alkyl esters have not been commercially acceptable, because the viscosity developed with same has been insufficient or slow to develop.
Phosphate alkyl esters may be mono-esters, di-esters or tri-esters. In the mono-ester, one primary mono-hydric alcohol, of C.sub.5 -C.sub.16 length is ester linked to a phosphate. A di-ester has two such alkyl alcohols linked to a phosphate. A tri-ester has three alkyl alcohols linked to it. As disclosed in Canadian Patent Application No. 2,216,325, commonly owned by the present applicant, commercially valuable gels are feasible with a di-ester content above 50%, preferably above 65%, and a tri-ester content below 5%. The remainder may be mono-ester.
In the aforementioned copending Canadian Application No. 2,216,325, gel development is enhanced in two important ways. First, the phosphate alkyl esters are neutralized with primary amines. Secondly, the applicants utilize a surfactant to enhance gel development. An appropriate surfactant is ammonium alkyl (C.sub.6 -C.sub.20) sulfate.
In the present invention, similar or increased gel development is accomplished by reacting the phosphate alkyl esters with a mineral acid, preferably sulfuric acid, before mixing the ester with the hydrocarbon being gelled. This step accomplishes two important purposes. First, the acid treatment tends to increase the dialkyl percentage of the ester, and secondly, the sulfuric acid reacts with the alkyl phosphate esters to form alkyl sulfates, which act as a surfactants, to assist in the subsequent cross link reaction.
The prior art, in particular U.S. Pat. No. 4,787,994 shows the use of sulfuric acid (or alternatively a low molecular weight sulphonic acid such as xylene sulphonic acid) with mono- or di-ester alkyl phosphates, to preferentially attract the alkali metal ions of the activator (such as sodium aluminate). This is disclosed as being effective to increase the available cross-linking sites in the alkyl phosphate ester, and thereby permit increased cross-linking by the aluminate ions. The prior art does not, however, disclose the use of sulfuric acid to increase the di-ester content of a solution containing mono-, di-, and tri-ester alkyl phosphates. Nor does the art show the formation of alkyl sulfates in situ. The present invention, moreover, does not utilize an alkali aluminate activator, but rather an iron citrate one, which is a departure from the prior art, in that the pH of the solution with an aluminate activator is in the 3.5-4.0 range, as opposed to the 1.0-2.0 range of the present invention.
Accordingly, it will be understood that the present invention represents a significant advance over the system described and claimed in Applicant's Canadian Patent Application No. 2,216,325, while sharing some part of the technology described herein. The present invention, moreover, is an advance over the technology of U.S. Pat. No. 4,787,994, in that it does not require the use of an alkali aluminate activator.
In order to break the gel of the present invention the preferred breaker is a pH adjusting breaker, such as calcium oxide or sodium carbonate. It is preferred that the breaker be encapsulated in a porous inert substance, such as nylon.
In a broad aspect, then, the present invention relates to a fracturing fluid for use in fracturing subterranean formations comprising: a hydrocarbon base; and acidified alkyl phosphate esters complexed with metallic cations, to form a gel, in said hydrocarbon base.